Traditionally among semiconductor elements, a power module for a power supply has a relatively large heating value, and it is necessary to select a base plate which allows preferable heat release performance of the entire circuit base plate preferable as a base plate on which the power module is mounted. Therefore, a base plate which uses an aluminum nitride (AlN) base plate having a high thermal conductivity as a ceramic base plate, and aluminum (Al) also having a high thermal conductivity as a metal circuit plate, is in practical use (refer to, for example, Patent Documents 1 and 2).    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H03-234045    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-171037
A power module 40 shown in FIG. 9 is known as an example. In this power module 40, two power module base plates 41 are joined on a heat-releasing object 31 by a brazing material 33 or solder. The heat-releasing object 31 is fixed on a water-cooled heat sink 37 by male threads 36.
Each power module base plate 41 is constituted so as to include an AlN/Al joint material 45 and a semiconductor chip 35. The AlN/Al joint material 45 is constituted by a circuit layer 43 made of aluminum being laminated on the first side of an insulated base plate 42 made of AlN (hereinbelow, referred to as an AlN base plate 42), and a metal layer 44 made of aluminum being laminated on the second side of the AlN base plate 42. The semiconductor chip 35 is mounted on the circuit layer 43 via solder 34. As the metal layer 44 or the circuit layer 43, 4N-Al (aluminum with purity greater than 99.99%) may be used to constitute the AlN/Al joint material.